This invention relates to logarithmic intermediate frequency (I.F.) amplifiers for use in radar receivers, and more particularly, to logarithmic I.F. amlifiers having duplicate amplifier stages in a parallel connection employed for extending the overall dynamic range of the amplifier for distinguishing a received amplified signal from background clutter signals.
Logarithmic I.F. amplifiers have been known in the past. A technique known as the Successive Detection Method which employs tandem detection stages for achieving logarithmic detection of I.F. signals is also well known. This technique is now the most common method utilized for building log I.F. amplifiers for military and commercial applications.
Generally, the logarithmic response is obtained by cascading linear I.F. amplifier stages, each having comparable quality limiting and detection characteristics. An approximation of a logarithmic video output signal is obtained by summing the contribution of each stage on a common summing line. The theory of operation is well documented in professional literature on the subject.
Commercially available logarithmic I.F. amplifiers designed to operate within the frequency range of 10 megahertz (MHz) to 1000 megahertz (MHz) can be readily purchased with such amplifiers having up to a 80 decibel (dB) dynamic range and a .+-.1.0 dB logarithmic linearity. The dynamic range is a measure of how much the received input signal power data can be compressed without deviating from the logarithmic relationship between the received input signal and the output amplified signal. This definition is based upon the general purpose of the log amp which is to compress or shrink for representation purposes a large input dynamic range into a much smaller output dynamic range.
Such a general purpose permits a large amount of gathered data to be processed by a method that improves the simplicity and utility of the gathered data. For example, if the received data were plotted on a linear milliwatt scale against the voltage output of the log amp, a very non-linear transfer function would result. However, if the axis representing the received power was converted to a logarithmic decibel scale having a conversion factor of 1 milliwatt equivalent to zero dBm, the non-linear transfer characteristic would be converted to a linear response. The range of the linear response of the characteristic curve is used to define the dynamic range of the log amplifier.
Thus, the range of linearity of the log amp provides predictability in the linear relationship between the received input power signals (dBm) and the output signal (volts) of the log amp. Because a dynamic range of 20 dB compresses a 100:1 input signal and a 40 dB dynamic range compresses a 10000:1 input signal, the log amp is very useful in identifying and amplifying weak radio signals.
However, the amplification and detection of a received signal is limited by the dynamic range of the log amp being utilized. Suppose a voltmeter employs a scale factor of 10 dB/volt such that a one volt change on the voltmeter output represents a 10 dB change in the input power. If on a first reading, the voltmeter indicates (+2) volts which translates into (-50) dBm on the linear transfer characteristic curve of the log amp and on a second reading, the voltmeter indicates (+4) volts which translates into (-30) dBm, a two volt output change has resulted from a 20 dB variation in the input power. By employing the scale factor of 10 dB/volt, the 20 dB or 100:1 power variation in compressed into a 2:1 output change on the indicator. This is the case since a 20 dB dynamic range provides data compression in the ratio of 100:1. The scale factor is derived from the slope of the linear region of the transfer curve, derived from the well known equation for a straight line EQU Y=mx+b (1)
where "m" represents the slope and "b" represents the "Y-axis" intercept.
A second parameter utilized as a measuring stick for the quality of a logarithmic amplifier is referred to as the log linearity. Such a measurement is defined as the amplitude of deviation of the actual characteristic within the dynamic range of the amplifier compared to the actual straight line of an ideal logarithmic response. This measurement addresses the accuracy of the linear characteristic and how far a distance measured in decibels the actual curve deviates from true log response. In this industry, a log linearity of .+-.1.0 dB is acceptable.
A further factor relates to the parameter referred to as the tangential sensitivity. The tangential sensitivity relates to a point located at the bottom portion of the log curve where the characteristic curve is no longer logarithmic. Beyond this point, the response is linear and not logarithmic and the transfer response is no longer a straight line. The point at which the logarithmic portion of the curve terminates and the linear portion begins is referred to as the lin-log crossover point.
A problem that continues to exist is that of the capability of the log amp to amplify and detect a received signal in the presence of high background "clutter." Clutter is generally defined as signals returned to a radar receiver (and the log amp located therein) which are reflected from objects of little interest. Examples of clutter include returned signals from the sea, land mass or even rain droplets. These returned signals may be very strong and tend to obscure signals returned from objects of interest such as aircraft. These signals are actually undesirable and in order for the log amp to recognize a returned signal of interest, such signal of interest must be stronger than the background clutter. The dynamic range of the log amp in decibels must be greater than the amplitude of the clutter to prevent the log amp from saturating and to permit detection of desired radio signals.
Therefore, to overcome this significant problem, a logarithmic amplifier with an extended dynamic range is necessary. This is the case because if the sea clutter in a specified vicinity has a magnitude of 80 dB, a log amp having only an 80 dB dynamic range cannot distinguish targets from the sea clutter in the specified vicinity.
Log I.F. amplifiers with extended dynamic ranges have not been developed in the past because the need for extended dynamic ranges greater than 80 dB had not previously been identified. However, the need to develop I.F. modules required to extend the dynamic range of, for example, a shipboard radar receiver was identified. In particular, in order to avoid saturation of the radar receiver in the presence of 80 dB sea clutter, extended dynamic range log amplifiers were required for detecting low flying targets. Specifically, low flying aircraft return or reflect low angle signals as a result of their position. The low antenna pointing angle required to detect these signals produce strong radar returns from the sea which tend to drive the logarithmic amplifier into saturation.
As an example, assume that the amplitude of a large clutter signal is -20 dBm. Under these conditions, the logarithmic amplifier output in volts is in saturation wherein this condition results in no additional output in volts being produced for an increased increment in the input power level in dBm. Even if a target of interest produces a signal having a power level of -10 dBm, the log amp cannot distinguish between the target and the clutter because the amplifier is in saturation. Therefore, the increased signal strength of 10 dB produces no additional incremental output signal in volts and is thus undetectable. It is necessary to provide a log amp having an extended linear range in order to detect targets having an amplitude greater than the sea clutter.
To achieve this goal, it was estimated that a log I.F. amplifier having a 105 dB dynamic range was required in order to operate the radar receiver with the system noise level set at least 15 dB above the lin log crossover point. With this in mind, a log I.F. amplifier having an 85 dB dynamic range from 25 MHz to 37 MHz had already been achieved. Such an amplifier had employed a plurality of cascaded stages in the Successive Detection Scheme.
In an effort to procure a 105 dB log I.F. amplifier from the commercial market, it was found that such an amplifier was not available. Different methods to extend the dynamic range of log I.F. amplifiers had been proposed but none had been implemented or since developed. In particular, an 85 dB log I.F. amplifier known in the past extended the dynamic range from -75 dBm up to +10 dBm employing the cascaded scheme. The limit level of the cascaded stages was on the order of -10 dBm, and the dynamic range had been extended by adding an attenuator and a unity gain limiter at the input. Further, the value of the first and last stage summing resistors were empirically selected for smoothing the logarithmic curve at the upper and lower ends of the curve. The noise figure of the amplifier had been degraded by the addition of the resistive attenuator at the input stage. Further, the 85 dB dynamic range obtained with this prior art amplifier was the highest logarithmic range achievable over the desired 20 MHz bandwidth using the Successive Detection Method.
Logarithmic amplifiers known in the past reflect a wide variety of design applications. An example included a solid state logarithmic amplifier and limiter device which employed seven logarithmic stages to achieve a 70 dB logarithmic range without utilizing vaccum tubes or diodes. Input voltage to the amplifier was attenuated and amplified in separate channels for producing seven logarithmic currents which were summed to produce the logarithmic amplified and limited output.
The invention consisted of several identical stages having output currents which were summed in parallel to provide a logarithmic output of a linearly increasing input signal. Logarithmic behavior over a wide range of input signals was obtained by attenuating the input voltage and amplifing the input voltage in separate channels, and then adding the logarithmic output currents. In addition, the gain of the amplifier was such as to provide a limited output over the desired input range which assisted in perserving phase information.
By inspection of the circuitry, it is noted that parallel channels are employed to extend the dynamic range of the amplifier. The channel employed for extending the dynamic range, normally referred to as the high level channel, is comprised of a cascade of attentuators each having a logarithmic current amplifier associated therewith. However, the low level channel positioned in a parallel combination with the high level channel includes only a logarithmic amplifier at each stage so that identical amplifying stages may not be employed in both the high level and the low level channels.
This particular invention permitted the logarithmic dynamic range to be extended to 70 dB which, by present standards, is modest. Further, this amplifier operates in the range of 1.1 MHz but does not indicate the maximum input power level that can be processed by the amplifier.
Another example of a logarithmic amplifier known in the past was a multistage, parallel summation, logarithmic video amplifier. This amplifier was capable of processing very short pulses in which linear amplifiers were utilized in each stage as a delay device such that all of the stages simultaneously provided an output signal in response to each input pulse. Typically, a parallel summation, logarithmic video amplifier of the past was multistage with each stage comprised of a linear amplifier, an attenuator coupled to the linear amplifier, and a logarithmic amplifier coupled to the attenuator. Each successive stage was also coupled to the linear amplifier of the preceeding stage while the outputs of all the logarithmic amplifiers were summed to form the response which approximated the logarithmic function of the input signal.
When the input signal was an ultrashort pulse, the typical logarithmic video amplifier was unable to provide a meaningful output because of the inherent delays in the system. Therefore, in order to overcome this problem, a delay line comprised of one or more linear amplifiers was selectively added to each stage so that the output of the stages was made to occur simultaneously in response to each input pulse. The number of linear amplifiers added to each stage was equal to the number of succeeding stages.
It should be noted that the logarithmic video amplifier of the second example is distinguishable from a logarithmic I.F. amplifier providing video detection and from a true logarithmic I.F. amplifier with no detection. It is sometimes difficult to distinguish between the structures, however, circuit details usually resolve any ambiguities. For example, circuits which employ operational amplifiers in the input signal path are usually not RF/IF devices and therefore imply video amplifiers such as the one described hereinabove. Further, the paralleling approach employed was not for the purpose of extending the logarithmic dynamic range for high level signals but was utilized for managing time delays.
A third example of amplifiers known in the past included a receiver employed for measuring the amplitude of input amplitude modulated signals which exceeded an 80 dBm range and which produced a digital output. The amplifier comprised a power splitter to which an input signal was applied via a limiter. First and second video detectors were coupled through the power splitter and in the case of the second video detector, an R.F. linear amplifier was connected in the signal path for providing an overall 40 dB of gain relative to the input of the first video detector. Each video detector comprised a detector stage having an output connected to a chain of amplifiers with each amplifier having a gain of ten. The outputs of the detector stage and the amplifiers were quantized and applied to a logarithmic analog-to-digital converter. The outputs of the respective converters were then applied to a combiner.
A recited objective of this invention was to provide a compact, less expensive amplitude measuring receiver which did not employ an analog logarithmic amplifier. This was accomplished by employing a receiver which exhibited two parallel channels with each channel responding to adjacent segments of the total dynamic range. It is noted that the range of each channel is scaled by a 40 dB amplifier instead of an attentuator. Further, the actual logarithmic conversion is accomplished by utilizing a signal video detector in each channel followed by a logarithmically weighted A/D converter instead of employing a cascaded succession of detection stages. Finally, the detected outputs are combined by employing a digital algorithm in lieu of continuous analog summation.
A fourth example of an apparatus known in the past included a radar receiver having as an objective to provide a simple radar receiver which included a radar display used for identifying storm centers. Particularly, the display would have a number of bright patches indicating storm regions with the bright patches having dark patches located within them for indicating the violent storm regions. This information permitted aircraft carrying such radar receivers to steer clear of such storm regions. A further objective of the radar receiver was to be readily arranged to vary the amplitude of the received radar signal at the point at which the brightness of the display began to decrease. This occurred with an increase in the amplitude of the received signal. In particular, a superheterodyne radar receiver was provided wherein the intermediate frequency stage split into two separate amplifying channels. The output signals of the channels were detected and combined in opposition to provide a display-intensity control signal. In order to obtain a greater dynamic range in the control signal, both channels exhibited logarithmic characteristics such that if the output signal was plotted on a linear scale against the input signal on a logarithmic decibel scale, a substantially straight line was obtained.
The receiver included a pair of channels in which a first channel was identified as including a first amplifier and a first detector while the second channel was identified as including the first amplifier, a second amplifier and a second detector. The two channels were combined in a parallel combination similar to parallel channels used in known log amplifiers. However, the major difference in the parallel channels employed in this fourth example was that they were combined in opposition for the purpose of obtaining a desired transfer response. Such a transfer response resulted in a reduction in the output parameter to high level signals and was not for the purpose of extending the overall dynamic range.
The fifth and final example included a logarthmic circuit which derived an output voltage proportional to the logarithm of a DC input voltage susceptible to wide variations in amplitude. The circuit included a constant current source which forward biased a diode such that the diode operated in the exponential portion of its voltage-current characteristic above the saturation current. The constant current source included first and second, cascaded feedback, DC operational amplifiers connected in a negative feedback circuit. An input terminal of the first amplifier was responsive to the input voltage and a circuit shunting the first amplifier output terminal included a resistor in series with the diode. The voltage across the resistor was sensed at the input of the second DC operational feedback amplifier. The feedback voltage derived from the output of the second amplifier was substracted by the first amplifier from the input voltage. This permitted the current flowing through the resistor to be proportional to the input voltage over a wide range of variations and amplitudes of the input voltage. This circuit was also a logarithmic video amplifier which did not provide intermediate frequency detection.
Hence, those concerned with the development and use of radar receiving systems in the surveillance field have long recognized the need for improved logarithmic I.F. amplifiers which extend the dynamic range of the amplifier for processing higher power input signals received from objects of interest with the dynamic range being extended at both ends of the transfer characteristic to a maximum peak not previously achieved. Further, the linearity of the logarithmic amplifier extended dynamic range must be maintained within a limited decibel range for minimizing the error of the logarithmic curve while simultaneously incorporating temperature compensation circuitry for improving the stability of the log amp curve over a variety of temperature ranges which minimizes the linearity error. Further, simplified circuitry is needed for promoting economy and commonality for permitting the employment of identical cascaded stages in both the high level and low level channels. The present invention fulfills all of these needs.